Field of the Invention
Embodiments of the present invention generally relate to energy storage systems and, more specifically, to increasing the demand reduction effectiveness of energy storage systems.
Description of the Related Art
As is well-known, utility companies typically generate electricity on-demand. Because utility companies do not store electricity for later use by customers, utility companies incur steep costs to deploy and maintain enough equipment to satisfy customer demand during demand peaks. Consequently, utility companies typically charge relatively large customers (e.g., commercial or industrial customers) an additional “demand” charge during these demand peaks. To determine the demand charge, a utility company tracks the average kilowatt (kW) demand of a customer over the course of a period of time, referred to herein as a “utility measure interval” (UMI). The utility company then sets the demand charge for the customer based on the highest average kW demand of all the UMIs included in a billing cycle.
As is well-understood, such demand charges can lead to significantly higher electricity costs for customers. For example, for a typical commercial or industrial customer, the demand charge may be between 30 and 70 percent of the total cost of energy over a billing cycle. To reduce the cost of electricity purchased from utility companies, some customers install energy storage systems on their premises that store energy in energy storage devices, such as large-scale batteries. Generally speaking, an energy storage system includes a system controller that attempts to discharge the energy storage device(s) when customer load is high and then charge energy storage device(s) when customer load is low. In so doing, a conventional system controller is configured to select and enforce one or more set-points. At any given time, the system controller attempts to configure the energy storage device(s) to discharge electrical energy at a rate that prevents the net load measured by the utility company from exceeding the relevant set-point during a particular UMI. In this fashion, the system controller attempts to constrain average kW demand of each UMI to a level that correlates with an acceptable demand charge.
One limitation of conventional system controllers is that they typically are unable to effectively manage average kW demand during a UMI when a “power limited event” occurs. A power limited event is when the customer load exceeds the capability of the energy storage device(s) to prevent the net load from exceeding the set-point during a given UMI. During UMIs that include power limited events, conventional system controllers may be unable to maintain the average kW demand of the UMI below the desired level. Such situations can occur when there is not enough time remaining in the UMI subsequent to the power limited event to discharge the energy storage device(s) long enough to bring the average kW demand back down to the set-point level. Thus, power limited events can oftentimes lead to unacceptably high demand charges that significantly impact the total cost of energy during a billing cycle.
For example, suppose that the power rating of an energy storage device is 100 kW and that the set-point during a UMI is 50 KW. Further, suppose that the customer load is 40 kW during the first 10 minutes of a 15 minute UMI and then 200 kW during the final 5 minutes of the UMI. During the first 10 minutes, because the customer load is below the set-point, a conventional storage controller does not discharge the energy storage device. During the final 5 minutes, a power limited event exists. Upon detecting the power limited event, the conventional storage controller immediately discharges all of the electricity stored in the energy storage device and requests the remaining required electrical energy (here, 100 kW) from the utility company. However, because there is only 5 minutes remaining in the UMI, there is not enough time left in the UMI to discharge the energy storage device long enough to limit the maximum average kW demand as measured by the utility company to 50 kW. More specifically, the maximum average demand for the UMI is 60 kW, and the demand charge associated with the billing cycle may increase dramatically.
As the foregoing illustrates, what is needed in the art are techniques for controlling energy storage devices to more effectively manage power limiting events.